This adds dynamic navigation of headers of the current page in the
sidebar. This is intended to help the user see what is on the current
page, and to be able to more easily navigate it. The "current" header is
tracked based on the scrolling behavior of the user, and is marked with
a small circle. This includes automatic folding to help keep it from
being too unwieldy on a page with a lot of nested headers.
This includes the `output.html.sidebar-header-nav` option to disable it.
I'm sure there are tweaks, fixes, and improvements that can be made. I'd
like to get this out now, and iterate on it over time to make
improvements.
